Wedge wire bonding is typically performed in hermetic type semiconductor devices to provide electrical interconnection between the actual semiconductor chip or die and the next level interconnection. The semiconductor die has bonding pads on its active surface. These bonding pads are connected to leads of a leadframe through wire bonds. The wire bonding is accomplished through the use of wire bonding equipment.
An integral part of the wire bonding apparatus is the bonding tool. FIG. 1 illustrates a typical wedge bonding tool 10 as known in the art. For ease of illustration, only a bottom portion of the bonding tool is shown. The bonding tool 10 has a front face 12, a bore 14, and a tool foot 16. A bonding wire (not shown) would feed through the bore 14 and under the wedge tool foot 16. The bonding tool itself is attached to an ultrasonic power source (not shown). In the wire bonding process, the wire is clamped between the tool foot 16 and the bonding pad on the semiconductor die. Ultrasonic power is applied to the wire which fuses the wire to the bonding pad. The tool foot is rectangular in shape and normally twice the wire diameter in length and 3 to 4 times the wire diameter in width. The minimum width of the bonding tool is limited by the bore 14 because the bonding tool must have sufficient mass to withstand the compressional and ultrasonic forces applied during the formation of the wedge bond. The tool foot geometry, the magnitude of the clamping force, and the ultrasonic power determine the final shape of the bond.
Bonds made in this manner are usually elliptical in shape--approximately twice the wire diameter in width and 3 to 4 times the wire diameter in length. The conventional arrangement of wedge wire bonds made in this manner is that the resultant wire bonds are perpendicular to the edge of the semiconductor die. This configuration of wire bonds is commonly referred to as orthogonal, wherein the angle that the wire makes with the die, the "incident angle," is defined as 0.degree.. Orthogonal wire bonding is usually performed sequentially around the periphery of the semiconductor die.
For semiconductor device packages which employ a fan-out pattern to the wire bonds, wherein the resultant wire bonds are not perpendicular to the die edge, wedge bonding with the known rectangular bonding tool can often result in damaged bonds. This configuration is known as non-orthogonal wire bond. The damage to the wire bonds takes the form of scores or scrapes on the bond caused by contact with the wedge tool foot 16 as an adjacent bond is being made. This contact damage can either occur between the front corner of the tool foot and the adjacent bond during the forward motion as the wedge bond is being made, or between the back corner of the tool foot and the adjacent bond during the reverse motion as the bonding tool is being moved after the bond is made. The damage to the adjacent bond can result in weakened bond strength and reduced device reliability. The bond damage can occur when the pitch between two adjacent bond-pads approaches the sum of 1/2 of the bond width and 1/2 of the wedge tool width, and non-orthogonal wire bonding is required. The minimum bond-pad pitch at which the wedge bond tool to touches an adjacent bond is referred to as the "contact distance". The higher the incident angle becomes, the more likely the wedge tool is to contact and damage adjacent bonds during bonding. Due to these geometric constraints, the pitch between two adjacent bond-pads must be increased as the bond angle increases. Another issue surrounding the occurrence of a wedge tool to existing bond contact is that during the contact the wedge tool is in the process of creating another bond. Since the bond being created is not being subjected to the proper forces, the bond may have inferior bond-to-pad adhesion attributes and thus lead to subsequent reliability problems.